1. Field
The present disclosure relates to a pulley pressure control apparatus for a belt-driven continuously variable transmission.
2. Description of the Related Art
Pulley pressure for a drive pulley and a driven pulley of a belt-driven continuously variable transmission is controlled for the following purposes: increasing or decreasing pulley-groove widths to change the transmission gear ratio and preventing slippage of an endless belt on the pulleys. Slippage of the endless belt is caused by an increase in torque transmitted between the endless belt and the pulleys. Thus, to prevent the slippage of the endless belt, pulley pressure is increased when the torque transmitted by the endless belt is large and decreased when the torque transmitted by the endless belt is small.
When driving wheels are braked on a road surface with a low coefficient of friction (hereinafter also referred to as a low μ road surface), the endless belt slips on the pulleys, easily causing lock-up of the driving wheels. Thus, the slippage of the endless belt needs to be prevented by making the pulley pressure larger on a low μ road surface than on a high μ road surface (hereinafter also referred to as a high μ road surface). However, excessive increase in the pulley pressure unnecessarily increases load on a hydraulic pressure generator such as an oil pump, and in turn, increases the fuel consumption of an engine that drives the hydraulic pressure generator.
Regarding this problem, Japanese Unexamined Patent Application Publication No. 2013-127287 aims to achieve both prevention of slippage of the endless belt and reduction in the load on the hydraulic pressure generator, by calculating pulley pressure necessary for preventing slippage of the endless belt on a high μ road surface and pulley pressure necessary for preventing slippage of the endless belt on a low μ road surface and employing the larger one of these pulley pressures in absolute value.